A conventional sheet-guiding device is disclosed, for example, by EP 0 306 682 A2. The device essentially comprises two blow strips to which blown air is applied and which are arranged upstream and downstream of the press nip formed between a blanket cylinder and a printing cylinder, over the cylinder width and parallel to the axis. The blow strip which is upstream in the conveying direction is arranged in a space above the incoming sheet between the blanket cylinder and printing cylinder. The blown-air stream is directed onto the blanket cylinder, into the printing zone itself and onto the upper side of the sheet carried in the grip of the grippers on the printing cylinder. The downstream blow strip, arranged downstream of the printing zone in the conveying direction, produces a blown-air stream which is directed onto the upper side of the sheet carried on the printing cylinder and onto the blanket cylinder, counter to the conveying direction. The reference primarily describes the sheet-guiding device during printing operation (print on position). Furthermore, in printing practice it is usual for the blown-air operation to be maintained when the blanket cylinder is thrown off (print off position), for example when checking the paper run or when a printing unit is not involved in the printing. The sheet printing material is then conveyed through the means of blown air (without contact with the inactive blanket cylinder).
According to DE 197 19 624 C1, a sheet-guiding device in a printing machine is known for guiding printing materials in the area of the blanket/plate cylinder and sheet-carrying cylinder when the blanket/plate cylinder is inactive. In this case, the blanket/plate cylinder, in the print off position, can be-positioned and fixed in position with a cylinder channel assigned adjacent to the circumferential surface of the sheet-carrying cylinder, it being possible for the blanket/plate cylinder to be stopped on the drive side by means of a clutch. Provided in the cylinder channels are sheet guide elements, which ensure the guidance of the sheet by mechanical and/or pneumatic means.
In the case of these pneumatically operated sheet-guiding devices, a disadvantage is that given the relatively high elasticity of the printing materials, such as for example in the case of board or sheet metal, the effectiveness of the sheet guidance is reduced. As a result of the relative movement with the blanket/plate cylinder stationary and the printing material being conveyed, the risk of smearing is increased, and as a result the print quality can be impaired.
UK patent GB 2 267 095 B discloses a varnishing device for a printing machine which is arranged downstream of the last printing unit. In the case of a varnishing unit which is not involved in the printing operation or not involved in the varnishing operation (the varnishing system is shut down), the contact between a freshly printed upper side of the printing material on the plate cylinder as it passes through the press nip can be prevented. For this purpose, the varnishing unit is constructed in two parts as a lower part and an upper part. The lower part accommodates the back-pressure cylinder and the upper part accommodates the plate cylinder with the varnish metering system. When the varnishing operation is shut down (print off position), the upper part, mounted in rotary joints on the lower part, is pivoted away from the sheet-carrying printing cylinder. This means that a relatively large distance between plate cylinder and printing cylinder can be achieved in the press nip (varnishing zone), and the sheet can pass through the varnishing unit without smearing without the use of pneumatic sheet-guiding means. If the varnishing operation is to be carried out again, the upper part is brought into contact with the lower part, and thus the plate cylinder is brought into contact again with the sheet-carrying printing cylinder (print on position). For this purpose, the previously uncoupled drive is re-engaged.